Gyrator circuit: simulate large coils electronically

Sometimes coils with large inductance is needed in telecommunication
systems and filters. Coils with inductance of many henries are not
usually very useful in modern small electronics circuits. To overcome
there is one solution which can be used in many applications: simulate
the behauvior of coils using electronics.

What's a gyrator ?

A gyrator converts an impedance into its inverse. This
lets you replace an inductor with a capacitor, a couple op amps, and some
resistors. A very handy gadget to have, especially if you're trying to put
large value inductors into a very small package.

Simple circuit

This simulation of coils can be done
in many ways, but the circuit below is maybe one of the simplest and
most inexpensive.

This circuit is one transistor gyrator circuit which can be
easily adjusted to simulate different inductance coils just
by changing the value of capacitor C2. The value of inductance
the circuit simulates is approximally simulates has value of
1 henry for every 2 microfarads in C2. So if you want to make
5 henry coil simulator, you just use 10 uF electrolytic.

The circuit needs some current passing through it to operate
correctly. The circuit is suitable fo example to be used as
telecommunication holding coil or line current feeding coil.

The idea for this circuit is from P. Strict and is was published in
Electronics World + Wireless World magazine September 1993 page 754.